Guys, yung mga jeep they can run the engine without radiator cap kasi yung mga tank nila ay oversize na yan compared to the cars. I worked in radiator company in Laguna (as design engineer) and at present nasa middle east ako same field pa rin. Yung sa kotse it was designed to be compact, kaya very limited lang yung volume ng coolant. Please note the normal boiling point ng plain water is 100 C at sea level. Talking about the coolant such Ethylene Glycol, yes it helps prevent corrosion, the same time lubricates the pump, and it will bring up the the boiling point of the coolant. But it will somewhat reduce the heat rejection on the radiator. Dont worry all the radiators were designed with the used of coolant (water+E.G.).